Wheel speed calculations are necessary to vehicle dynamic control systems, such as brake control strategies exemplified by antilock brake control, traction control and stability control. The performance of many of these control methods is significantly improved by precise wheel speed calculations. For example, methods that are enhanced by fast response to wheel speed signal accelerations are sensitive to small errors in the wheel speed calculation, and require highly accurate, precise wheel speed calculations.
Wheel acceleration calculations are typically made by taking two wheel speed calculations and dividing by the time interval between the wheel speed calculations. As the time interval is reduced for fast response, small errors in the wheel speed value have an even larger affect on the acceleration calculation.
A significant source of error in the wheel speed calculation is variation in the wheel-speed tone ring or magnetized encoder that modulates the magnetic field used to calculate wheel speed. A typical wheel speed sensor has teeth on a tone-ring, providing a number of magnetic field changes per rotation of the wheel. However, slight variations in manufacturing tolerances for the teeth, broken or missing teeth, imperfections in a tire, or deformation of the tone-ring when high-loads are present on the vehicle may result in different amounts of rotation relating to errors in wheel speed calculations. As described above, small differences in wheel speed signals are significant in the calculations of wheel speed and acceleration.
There is a need to minimize wheel speed and acceleration errors in order to maximize the performance of dynamic control systems on a vehicle, particularly antilock brake control.